Field of the Invention
The present invention relates to an ophthalmologic apparatus and an imaging method.
Description of the Related Art
Optical coherence tomography (hereinafter referred to as OCT) becomes commercially practical as a method for non-destructively and non-invasively acquiring a tomographic image of a measurement object such as a living body. The OCT can acquire information concerning tomographic images of the retina in the fundus of an eye to be inspected particularly in the ophthalmologic field, and is widely used in an ophthalmologic diagnosis of the retina, and the like.
In the OCT, light reflected from the measurement object interferes with reference light, the time dependency or wave number dependency of the intensity of the light that has interfered is analyzed, and thereby information concerning the tomographic images can be obtained. As for an optical coherence tomographic imaging apparatus (hereinafter referred to as an OCT apparatus) using such an OCT, a time domain OCT apparatus is known which obtains depth information of the measurement object by changing the position of a reference mirror. In addition, a spectral domain OCT (SD-OCT: Spectral Domain Optical Coherence Tomography) apparatus using a broadband light source is also known. Furthermore, a wavelength swept-OCT (SS-OCT: Swept Source Optical Coherence Tomography) apparatus is also known which uses a wavelength variable light source device that can change an oscillation wavelength as a light source. Incidentally, the SD-OCT and SS-OCT are collectively referred to as Fourier domain OCT (FD-OCT: Fourier Domain Optical Coherence Tomography).
In recent years, an angiography using this FD-OCT has been proposed and is called as OCT Angiography (hereinafter referred to as OCTA).
The fluorescence contrast imaging which is a common angiography in modern clinical medical care requires injection of a fluorescent dye (for instance, fluorescein or indocyanine green) into the body, and two-dimensionally displays a vessel which is a pathway of the fluorescent dye. However, there are side effects on the contrast medium, there are also cases where the contrast medium causes nausea, rash and cough and rarely causes shock symptoms, and thus fluorescence imaging involves a risk. On the other hand, the OCTA enables non-invasive angiography, and can display a vascular network three-dimensionally. Furthermore, the OCTA can visualize microvessels of the fundus when being used in the ophthalmologic diagnosis or the like, and accordingly is attracting attention.
Here, the OCTA is a technique of specifying a vessel region by repeatedly (a plurality of times) scanning the same site of a measurement object and extracting a difference between the signals that are acquired by each scan. Incidentally, in the present specification, it is called as cluster (group) scanning to scan the same site of the measurement object a plurality of times, which corresponds to the same scanning line, and a group of a plurality of scans that are included in the cluster scanning is referred to as a scanning group. On the other hand, it is called as single (single) scanning to scan the same site of the measurement object only once, which corresponds to the same scanning line.
In the OCTA and general OCT, it is desired that the apparatus tracks the movement of the measurement object (tracking) and corrects the scanning position (hereinafter referred to as tracking operation), when two-dimensionally scanning the surface of the measurement object. Regarding this, in the field of the OCT, it is known that the apparatus performs the tracking operation for each main scanning (B-scan) in two-dimensional scanning, with respect to the movement of the measurement object by reason of a fixation failure or the like of an eye to be inspected which is the measurement object (Japanese Patent Application Laid-Open No. 2013-154189).
However, in the OCTA, the time period which is needed for one cluster scanning is limited, and there is a case where it is not appropriate to apply the above tracking operation for each main scanning as it is in the general OCT. In other words, in the OCT apparatus, there is a case where the timing at which the apparatus should correct the scanning position in the tracking operation varies depending on an imaging mode such as an imaging mode using the OCTA (OCTA mode) or an imaging mode using the general OCT (OCT mode).